Most of us know fluoride as the stuff that's in our toothpaste. Fluoride is a molecule containing the element fluorine (F) -- and you get it when you combine fluoride with another element. The fluoride added to water is usually either hexafluorosilicic acid (H2SiF6) or sodium hexafluorosilicate (Na2SiF26). But fluoride occurs naturally in all sorts of places, including some tea leaves, seaweed, soil and rocks. It's put in toothpaste and mouthwash to prevent tooth decay, but it's also added to rat poison and insecticides to kill stuff.

Most fluoride toxicity cases occur when someone ingests such poisons or insecticides. It's one of the reasons people child-proof their homes. When someone (typically a child) swallows fluoride, it hits­ the stomach and forms hydrofluoric acid, which can irritate and corrode the stomach lining. Once the gastrointestinal (GI) tract absorbs the fluoride, it causes, at the very least, bad stomachaches and diarrhea. It can cause serious GI problems and sometimes even seizures at doses of above 3 mg/kg. At doses of 5 to 10 grams for an adult, or 500 mg for a child, fluoride poisoning can damage major organs and cause death.

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But that's not the kind of fluoride poisoning people are talking about when they discuss fluoridating water. They're talking about a less immediate form of fluoride toxicity that can result from drinking over-fluoridated water over a significant period of time -- like years. In this type of fluoride poisoning, the most commonly reported effects are dental and skeletal fluorosis -- conditions that weaken the teeth and bones, ironically.

The thing is, our drinking water already has fluoride in it, even without adding a thing. In drinking water, you'll find naturally occurring fluoride in amounts ranging from 0.05 parts per million (ppm, or mg/L) and 14 ppm [source: Bender]. The EPA set the highest safe amount at 4 ppm, so that upper-level 14 ppm is considered dangerous. The reason some states and countries add fluoride is because the low end of natural occurrence -- that 0.05 ppm -- is considered to be too low to help strengthen teeth. About 60 percent of U.S. cities fluoridate public drinking water to achieve levels of about 1 ppm. According to the American Dental Association, the optimal dose varies from 0.7 ppm to 1.2 ppm [source: ADA]. The U.S. National Academy of Sciences even recommends a daily intake of 1.5 mg to 4 mg a day [source: Drugs.com] to support healthy teeth and bones.

So if the right amount of fluoride has such appealing health benefits, and we can control the amount of fluoride in drinking water, why has most of Europe rejected water fluoridation? In the next section, we'll see what the big debate is all about.